There is known in the prior art (cf. R. Ya. Karklin, A. K. Probok. Biosynthesis of organic acids. Riga, 1972) a method of producing citric acid comprising the steps of preparing a seeding material of the strain Aspergillus niger -119, fermentation under production conditions by cultivating spores (conidia) of said strain on a molasses-containing nutrient medium, and isolation of citric acid from the fermented broth. The strain Aspergillus niger -119 (USSR Inventor's Certificate No. 170,898, published 11.05.1965) has the following morphological and cultural properties: A five-day-old culture grown on the Czapek-Dox broth has conidial heads round or starlike in shape, 85 to 190 mcm in diameter, double-layer sterigmata, with sterigma lengths being 11 mcm in the first order and 7.5 mcm in the second order, round conidia, 3.4 mcm in diameter, and conidiophores 0.7 to 0.5 mm long. A five-day-old gigantic colony grown on the Czapek-Dox broth is round, 40 mm in diameter; the substrate mycelium is radially pleated, the aerial mycelium is high, the conidia are dark brown, and the asporogenic zone is 7 mm. When grown on must agar, a five-day-old gigantic colony is round, fleecy, 73 mm in diameter, with an asporogenic zone of 11 mm. On a molasses medium, the strain Aspergillus niger -119 provides a citric acid yield of between 70 to 75%, calculated with reference to the sugar content of the molasses used. On must agar, the yield of spores (conidia) per 1 dm.sup.2 is 0.9 g. Conidia content is 15 to 20 billion per gramm.
The use of the strain Aspergillus niger -119 in citric acid production gives a low spore yield at the stage of preparing the inoculum, with low biochemical activity characteristics.
Amongst the methods of preparing seeding materials for citric acid production to be found in literary sources, the closest to the item applied for herein is the method described in the USSR Inventor's Certificate No. 568,677 (published 15.08.1977). In this method, a seeding material of the strain Aspergillus niger R-1 is obtained by seeding a pure culture of the strain on a must agar nutrient medium of the following composition, in percent by mass:
______________________________________ Sugar as beer must 6 to 8 Urea 0.05 to 0.1 NaCl 1 to 2 CoSO.sub.4 0.0001 Agar 2 to 3 ______________________________________
The pH of the medium is maintained at a level of 5 to 6.
Cultivation is carried out at a temperature of 30.degree. to 32.degree. C. for 9 to 10 days in cuvettes sized 10 to 12 dM.sup.2. Ripe conidia are separated from the mycelium with the aid of a brush or a vacuum device. The yield of conidia per 1 dm.sup.2 spore growth area is 1.0 to 1.1 g. Conidia content is 24 billion per gramm.
The strain Aspergillus niger R-1 has the following morphological and cultural properties: A five-day-old culture grown on the Czapek-Dox broth has conidial heads of round shape, 205 to 215 mcm in diameter, vesicles of oblong shape, 27 mcm in diameter, single-layer sterigmata, 10 mcm long, round conidia having a mean diameter of 7 mcm, and conidiophores 3 to 4 mm long and 20 mcm in diameter. A five-day-old gigantic colony grown on the Czapek-Dox broth is round, 45 to 50 mm in diameter; the colony margin is smooth, the asporogenic zone is 6 mm, the substrate mycelium is radially pleated, the sporophores are of medium density, the conidia are dark brown. When grown on must agar, a five-day-old gigantic colony is 90 mm in diameter, with sparse long sporophores in the centre of the colony, surrounded by a belt of abundant short sporophores which become sparse towards the margin.
On molasses media, with the superficial method of cultivation, the strain Aspergillus niger R-1 provides a citric acid yield of 99 to 100% based on the sugar content of the molasses used.
The strain Aspergillus niger R-1 as used in the production of citric acid is deficient in that it gives a low yield of spores (conidia) per unit spore growth area at the inoculum preparation stage and a low spore content per unit mass.